Technical Field
[0001] The invention relates to methods of controlling the precipitation or depositing of
particles from a solution in a cell in which an electric field is applied between
an anode and a cathode, in particular the precipitation of metal compounds such as
oxides and hydroxides and the coprecipitation of mixed metal oxides from a solution
of the metal salt(s) in an electrolysis cell, but also the electrolytic deposition
of cathodic metal deposits as well as the electrophoretic deposition of colloidal
particles.
Background Art
[0002] Methods of electrolytically co-precipitating mixed metal oxides are known from Canadian
Patent 593.187 and UK Patent 864.249. Another method of precipitating or co-precipitating
metal compounds, more particularly oxides and hydroxides, in an electrolysis cell
is described in Canadian Patent 623.339.
[0003] It was recognized in the latter patent that the process should be carried out at
a specific and as constant as possible pH, and this was achieved by carrying out a
preliminary electrolysis to obtain suitable starting conditions and setting the electrolysis
current at a value corresponding to the rate of supply of a solution of the salts
from which the compounds are precipitated. In carrying out these known processes,
it has been observed that the particle size and density do not remain constant and
this reduces the usefulness of the product. The invention is based on the observation
that the uneven particle size and variable density obtained with the known processes
are due to variations in the solution pH which occur when following the prior teachings
and because of the inherent difficulties in measuring and controlling the pH in an
electrolysis cell particularly when gas is bubbled in or is electrochemically generated.
[0004] Similar considerations apply to the control of the particle size and density of electrodeposited
coatings and to the control of electrophoretic deposition.
Disclosure of the Invention
[0005] According to the invention, the precipitation or the depositing of the particles
is controlled by measuring the pH of the solution in the cell using a probe shielded
from migrating electric current and adjusting the pH of the solution to a selected
value as a function of the measured pH.
[0006] Preferably, this is achieved by supplying a first electrical signal representative
of the measured pH, comparing the first electrical signal with a reference electrical
signal corresponding to the selected pH, and adjusting the pH of the solution as a
function of the difference of the first signal and the reference signal to maintain
it close to the selected pH.
[0007] The pH may be adjusted by acting on any pH-determining parameter, such as by bubbling
acid vapour or base vapour into the solution or by adjusting the supply of air, in
the case where air is supplied as an oxidant. This method of bubbling in a vapour
or air is particularly appropriate when a solution of metal salt(s) to be precipitated
is obtained by dissolving at least one metal anode.
[0008] In another arrangement, the pH is adjusted by varying the electrolysis current. For
example a preprepared solution of metal salt(s) is introduced into a compartment of
the electrolysis cell in which the particles are precipitated, and the ions liberated
by the salt(s) are passed through a separator into another compartment of the cell
by passing an electrolysis current at a rate controlled as a function of the measured
pH to keep the pH of the solution at the selected value. This arrangement is recommended
when it is desired to incorporate a metal such as barium which does not dissolve anodically
or manganese which is soluble without the passage of current so that, when connected
as an anode, it does not dissolve in proportion to the current passed. However, the
arrangement can of course also be used to supply the salts of other metals.
[0009] The solution of salts can be introduced into the cell compartment by dripping it
in uniformly or by blowing in a fine dispersion of the solution for example with an
oxidizing gas, or in any other suitable manner.
[0010] It is possible to combine this arrangement with the previous one by providing ions
of at least one further metal to be precipitated by dissolving at least one metal
anode, and adjusting the pH of the solution by bubbling acid vapour, base vapour or
by adjusting the supply of air into the solution when said controlled electrolysis
current reaches a threshold value.
[0011] It is equally possible, when a preprepared solution of the metal salt(s) is supplied
to the electrolysis cell, and whether or not this is combined with dissolving at least
one metal anode, to supply a substantially constant electrolysis current calculated
to correspond more or less exactly to the rate of supply of metal salt to the solution
so that ions liberated by the salt(s) are passed through a separator into another
compartment of the cell at a corresponding rate, and to adjust the pH to the selected
value by bubbling in acid vapour, base vapour or by adjusting the supply of air as
a function of the measured pH.
[0012] Advantageously, to avoid disturbance of the pH measurement in the electrolyte solution,
the pH is measured by a probe disposed in a tube which shields the probe from gas
bubbles and stray currents carried by migrating ions. This may be a tube which extends
down to below a source of gas bubbles, or the end of the tube may be closed with an
electrolyte-permeable but gas-bubble impermeable gauze. In some instances, instead
of a tube, a U-shaped section or even two parallel spaced plates may be sufficient
to shield the probe from the influence of stray current carried by migrating ions.
[0013] In most cases, an oxidizing gas is supplied to the electrolyte solution to precipitate
an oxide or mixed oxide of the metal or metals. It is also possible to electrochemically
generate an oxidizing agent in situ in the solution using an insoluble anode situated
in the precipitating zone, possibly operating in conjunction with soluble anodes and
preferably with an electrolyte such as sodium sulphate suitable for the generation
of oxygen. Alternatively, hypochlorite can be generated as the oxidizing agent by
using a sodium chloride or potassium chloride electrolyte. When an auxiliary anode
is used to generate an oxidizing agent in situ, it is possible to control the pH of
the electrolyte by adjusting the current supplied to the auxiliary anode as a function
of the measured pH.
Brief description of Drawings
[0014]
Fig. 1 is a schematic representation of an electrolysis cell and associated equipment;
Fig. 2 is a circuit diagram of a control unit; and
Figs. 3 and 4 are schematic representations of further arrangements.
Best modes for carrying out the Invention
[0015] As shown in Fig. 1, an electrolysis cell comprises a housing 1 containing an electrolyte
2 in which a cathode 3 and anodes 4 and 5 dip. The cathode 3 is connected to the negative
terminal of two D.C. power sources 6 and 7 whose positive terminals are connected
to respective anodes 4 and 5 so that these anodes can be supplied at different voltage
and hence different current density. The cathode 3 may, for example, be of iron and
the anodes 4, 5 of iron and nickel respectively, and the electrolyte may be an aqueous
alkali metal salt solution such as KC1 in which the anode metals dissolve proportionately
to the respective currents that they pass. It is understood that two anodes are shown
by way of example; any convenient number of anodes of different metals could be used,
each supplied with current to dissolve at a desired rate, or a single anode consisting
of a metal (e.g. iron) or an alloy (e.g. iron/nickel) could be used.
[0016] Under the anodes 4, 5 extends an air tube 8 through which air is delivered from an
air pump 9, this air serving to oxidize the dissolved metals which are co-precipitated
in the electrolyte as a mixed oxide powder, e.g. a nickel ferrite powder when iron
and nickel anodes are used. Other oxidizing mediums could be used, such as oxygen,
ozone, chlorine, hypochlorite, hydrogen peroxide and so on. It is also possible to
generate an oxidizing agent by electrolysis.
[0017] As shown, preferably the air pump 9 is connected to the tube 8 via a jar 22 advantageously
containing filter material 23, for example glass beads, a second jar 24 containing
25% KOH 25 and a flow meter 28 which is set to control the rate of supply of air by
the pump 9 according to the quantity of particles to be kept in suspension in the
oxidizing zone of the electrolyte 2. The KOH in jar 25 removes carbon dioxide from
the air and prevents the unwanted formation of ferrous carbonate in the housing 1.
[0018] The purpose of the filter material 23 is to prevent fumes from reaching the air pump
9; however, in installations with a large jar 22 the filter material 23 may be omitted.
Also, if desired another similar filter jar may be connected between the jar 24 and
tube 8.
[0019] The tube 8 may simply be an open-ended tube but it is preferable in larger installations
to use a distributor 27 perforated in its upper face with perforations of a selected
size which produce relatively fine or large bubbles depending on the quality and dimensions
of the powder being produced.
[0020] In the bottom of the housing 1 is an optional magnetic stirrer 26, it being understood
that in instances where very magnetic powders are being produced, mechanical stirring
means will be preferred.
[0021] The pH of the electrolyte 2 is measured by a glass probe 10 which is inserted in
the electrolyte 2 and is sheltered from the migrating electric current between the
electrodes 3, 4 and from rising gas bubbles by being enclosed in a tube 10A having
at its lower end a gauze 10B of suitable non- metallic material such as PTFE or PVC
which is permeable to the electrolyte but not to the gas bubbles. In this manner,
the probe 10 accurately measures the pH of electrolyte ascending by convection in
the tube without disturbance from air bubbles or stray current carried by migrating
ions. This probe 10 is connected to a pH meter, which provide a visual display of
the measured pH and an analog electric output signal 12 corresponding to the measured
pH. The signal 12 is delivered to a control circuit 13, described in detail later,
which compares the signal 12 with a reference signal corresponding to a selected pH,
and switches on or off an air pump 14.
[0022] When actuated, the air pump 14 blows air through a tube 15 which extends into a jar
16 containing filter material 17, for example glass beads, into which the tube 15
dips. This filter material 17 prevents acid fumes from penetrating into the air pump
14. The top part of filter jar 16 is connected by a tube 18 to another jar 19 containing
concentrated hydrochloric acid 20, into which the tube 18 dips. Finally, a tube 21
extends from the space in jar 19 above the hydrochloric acid 20 to the electrolyte
2 in vessel 1, the tube 21 terminating just below the space between cathode 3 and
anodes 4, 5. If desired, this tube 21 may be fitted with a perforated distributor
like distributor 27.
[0023] Thus, when the air pump 14 is switched on, the air blown along tubes 15 and 18 drives
air containing acid vapour along the tube 21 and this acidified air is delivered into
the electrolyte 2 and bubbles up through the zone where the co-precipitation of the
metal oxides is taking place. The, air delivered by tube 21 thus acts in the same
way as the air delivered by tube 8 to oxidize the metal ions. The minute particles
of acid delivered are very evenly distributed by the bubbling air in the co-precipitation
zone of the electrolyte 2 and/or on the co-precipitated particles, and this ensures
a very homogeneous distribution of the acid in the co-precipitation zone. This supply
of acidified air continues until the pH has dropped to the pre-selected value, corresponding
to a desired particle size.
[0024] With reference to Fig. 2, the pH control circuit 13 is connected to the A.C. mains
by a transformer 30 which provides, by means of diodes Dl and D2 and capacitors C1
and C2, a stable D.C. input powering an operational amplifier 31. The output signal
12 of pH meter 11 is connected to a resistance bridge formed by resistors Rl and R2
and to one input, 32, of the operational amplifier 31. The input signal on 32 is thus
a voltage which fluctuates in proportion to the measured pH. The other input 33 of
amplifier 31 is connected to a potentiostat PI which provides an input signal on 33
of constant voltage, but which can be set at any value corresponding to a selected
pH according to the desired particle size and density.
[0025] It can readily be seen that by setting the potentiostat PI appropriately in relation
to the values of resistors Rl and R2, the signal on 33 can, if desired, be set equal
to the signal on 32 and in fact this is what is usually done at the beginning of operation:
the pH of the electrolyte 2 is brought to a selected value according to the desired
particle size, and the potentiostat Pl is set to a corresponding pH value.
[0026] Via a resistor R3, a transistor 34 and a relay 35 stabilized by a capacitor C3, operational
amplifier 31 controls a switch 36 controlling the air pump 14. As shown, the switch
36 controls two circuits for actuating the pump: one circuit for use when the pump
14 actuates the supply of acid or air, the other circuit for use when the pump 14
actuates the supply of a base. In the instance described with reference to Fig. 1
where there is acid 20 in the jar 19, the circuit corresponding to the supply of acid
is used. In this case, as long as the signal on 32 does not exceed the reference signal
on 33, the amplifier output is zero, switch 36 remains open and the air pump 14 is
off. However, when the pH of the electrolyte 2 rises so that the signal on 32 exceeds
the reference signal on 33, this triggers the amplifier 31 and the switch 36 is closed
(as shown) by energisation of the coil of relay 35. As soon as the pH of the electrolyte
2 is brought back to the reference value by the supply of acidic air, the amplifier
output drops to zero and the switch 36 springs open.
[0027] With the described pH measuring and control system, the pH can be controlled very
accurately, to about +/- 0.05, and because the pH of the precipitation zone can be
maintained homogeneous at a chosen pH, particles of a specified uniform size and density
can be obtained.
[0028] Fig. 3 shows an arrangement in which a solution of salts for conversion to a mixed
oxide is dripped into an electrolysis cell from a burette 40. The cell has a housing
41 containing an electrolyte 42 such as KC1 or NaCl. The cell housing 41 is placed
on a heater 43. Cell housing 41 is divided by a diaphragm or membrane 44 into an anode
compartment containing an anode 45 and a cathode compartment containing a cathode
46. The anode 45 is preferably a dimensionally-stable anode consisting for example
of a sheet of expanded titanium mesh coated with an electrocatalytic coating such
as a so-called "mixed crystal" of ruthenium oxide-titanium oxide. The cathode 46 may
consist of iron, nickel or stainless steel. The diaphragm or membrane 44 is of the
anion-exchange type, i.e. in the case where a mixture of chlorides is supplied by
the burette 40 it will allow chloride ions to migrate from the cathode compartment
into the anode compartment.
[0029] Into the cathode compartment extends a tube 47 through which air can be supplied
in the vicinity of cathode 46.
[0030] The arrangement so far described corresponds substantially to the prior-art arrangement
of Canadian Patent no. 623.339. According to the practice of the prior art, current
was passed between the anode 45 and cathode 46 to bring the pH in the anode and cathode
compartments to desired values. Then a solution of metals to be precipitated, for
example, a mixture of FeC1
2, NiCI
2 and MnC1
2 in a desired ratio, is introduced into the cathode compartment from the burette 40,
and the current and the rate of supply of the solution were set so that the concentration
of precipitating ions in the cathode compartment remained substantially constant.
Thus chloride ions from the supplied chloride solution migrate through the diaphragm
or membrane 44 and are released as chlorine gas at the anode 42. In the cathode compartment,
the dissociated iron, nickel and manganese ions are oxidized by the air supplied through
tube 47 and are precipitated as a mixed oxide. The starting-up of this process is
complicated, and any fluctuations in the current supplied or the rate of supply of
the solution can upset the equilibrium of the process so that very careful monitoring
is required.
[0031] According to the invention, the arrangement is provided with a pH probe 48 dipping
into the cathode compartment and shielded in a tube 48A with an electrolyte-permeable
but gas-impermeable PTFE or PVC gauze 48B. This tube protects the probe from the effect
of stray current carried by migrating ions and from the ascending gas bubbles, so
that the pH measurement is not disturbed. The probe 48 supplies an analog signal proportional
to the measured pH to a pH control circuit 49 which compares this signal with a pre-set
control value corresponding to a desired pH. The circuit 49 in turn supplies an analog
output signal to a DC source 50. This DC source is connected to the anode 45 and cathode
46 so that the electrolysis current supplied is exactly that which required to maintain
a constant pH in the cathode compartment, whatever may be the fluctuations in the
rate of supply of the mixed chloride solution from burette 40. Start-up of the co-precipitation
process is thus greatly facilitated and optimum conditions for co-precipitation at
an exactly set pH can be maintained easily.
[0032] Fig. 4 shows a hybrid arrangement which produces mixed oxides by the combined action
of dissolving one or more soluble anodes (as in Fig. 1) and supplying a solution of
at least one metal salt (as in Fig. 3). The cell of Fig. 4 has a housing 61 containing
an electrolyte 62 such as KCl or NaCl, the housing 61 being placed on a heater 63.
The cell housing 61 is divided by an anion-exchange diaphragm or membrane 64 into
a first compartment containing a dimensionally-stable anode 65 and a second compartment
containing a cathode 66, for example of iron, and two auxiliary soluble anodes 67
and 68, for example of nickel and iron. Into this second compartment extends a tube
69 through which air or an air/ammonia mixture is supplied and bubbled under the cathode
66 and anodes 67, 68 through a perforated distributor 70. Above the second compartment
is a burette 71 from which a solution of one or more salts, for example a solution
of manganese chloride, can be supplied adjacent to the cathode 66. In the zone of
electrolyte 62 between or adjacent to the cathode 64 and soluble anodes 67, 68 dips
a glass pH probe 72 enclosed in an open-ended glass tube 73 which extends to below
the distributor 70. This tube 73 protects the pH probe 72 from the effect of stray
current carried by ions migrating between the cathode 66 and anodes 67, 68, and from
the effect of ascending gas bubbles. The probe 72 thus accurately measures the pH
of electrolyte ascending by convection in the tube 73.
[0033] The probe 72 supplies an analog signal proportional to the measured pH to a control
circuit 74 which compares this signal with a pre-set control value corresponding to
a desired pH. The circuit 74 in turn supplies an analog output signal to a DC source
75 connected to the anode 65 and cathode 66 so that the electrolysis current supplied
is exactly that which is required to maintain a constant pH in the second compartment.
[0034] As in the arrangement of Fig. 1, the anodes 67, 68 are connected to separate DC supply
sources with a common cathodic connection with source 75 and cathode 66, so that the
anodes 67, 68 can be supplied at different current densities and dissolve proportionately
to the different currents that they pass. The rate of supply of MnC1
2 solution from burette 71 is correlated to the current supplied to anodes 67, 68 and
hence their rates of dissolution so that a mixed oxide (manganese-nickel ferrite)
of desired composition is obtained.
[0035] In normal operation, carbon dioxide-free air is supplied via tube 69 to ensure oxidation
of the dissolved metals. However, as a modification of the process, it is also possible
to intermittently supply an air/ammonia mixture as an additional means of keeping
the pH in the second compartment constant. This can be achieved using the circuitry
of Fig. 2 with the switch 36 controlling the circuit for the supply of a base (ammonia).
Or the pH control circuit could be arranged so that when the analog signal of circuit
74 (and hence the current between anode 65 and cathode 66) reaches a threshold value,
the supply of air/ammonia is actuated.
Example I
[0036] The arrangement of Figs. 1 and 2 was used for the production of a nickel ferrite
using nickel and iron anodes, the anode currents being arranged to dissolve the metals
to give a theoretical composition of equimolar Fe
20
3/NiO. The electrolyte was 5% KCl at 70°C. The electrolysis was carried out both with
control of the pH between 6.2 and 6.4 by the intermittent supply of acid air using
the described pH measurement and control device and, for comparison, using the prior
art method without servo-control of the pH in the cell. The powders obtained were
dried and examined by transmission electron microscopy. The measured particle size
ranges of greater than 50% of the particles (PSR 50) and the total particle size ranges
(TPSR) are shown in Table I.
[0037] It can readily be seen from this table that the particles/flakes produced are considerably
finer and more uniform when the pH is controlled according to the invention.
[0038] It was further observed that when the pH control device was used, the intermittent
supply of HCl vapour continued throughout the process with variable intervals.
[0039] Also, the pH range of 6.2 - 6.4 was chosen as this is found to give the most dense
nickel ferrite. When a less dense product is required, the pH is set at a selected
higher value.
Example II
[0040] The arrangement of Figs. 1 and 2 was used for the production of iron oxide (ferrite)
using a single soluble iron anode in a 5% KCl electrolyte. The pH was controlled at
6.2 - 6.4. The process parameters were varied by using the electrolyte at 70°C or
90°C and by blowing in very fine air bubbles or larger air bubbles, or with in situ
oxidation using an auxiliary dimensionally stable electrode.
[0041] In all instances, it was found that the pH control circuit operated to intermittently
supply HCl vapour at varying intervals during a start-up phase of about 30 minutes
to 1 hour and thereafter the process reached a steady state at constant pH with little
or no further additions of HCl vapour.
[0042] Also, in all instances, a fine uniform powder was obtained. Typical size ranges measured
by transmission electron microscopy are: PSR 50 : 0.03 - 0.1 µ, TPSR: 0.03 - 0.5 µ.
Furthermore, the particles were analysed for their contamination with potassium. The
average potassium content was 45 ppm with a maximum of 59 ppm and a minimum of <5
ppm.
[0043] When it was attempted to carry out the same process with the pH control device switched
off, it was found that the pH progressively rose from 6 to about 9 and instead of
fine particles a sludge was produced. Analysis of the product showed a potassium contamination
of the order of 1900 - 2000 ppm.
[0044] Also, when the process was carried out at about 75°C or below, it was found that
the pH control could be achieved by blowing in an extra controlled amount of air (freed
from CO
2 by passing through KOH) without acid vapour. At higher temperatures the acid vapour
is needed to compensate for evaporation of acid fumes from the cell.
Industrial Applicability
[0045] The particles obtained by precipitating at controlled pH according to the invention
will often be oxides which, on account of their uniformity and extremely fine size,
are useful in many applications especially as permanent magnets and in electronics
applications for ferrite-based materials.
[0046] One particular application for the fine and uniform particles obtained according
to the invention is in the manufacture of electrodes for electrolytic processes, such
as the ferrite-based electrodes for molten salt electrolysis disclosed in PCT publication
WO 81/01717. In manufacturing such electrodes, the particles obtained according to
the invention are usually cold pressed isotatically then sintered at an elevated temperature
(about 1350°C) under argon or with a low oxygen pressure. The uniformity and fineness
of the particles considerably simplifies the sintering operation and gives very high
density electrode bodies (more than 90% of the theoretical density). The particles
can be sintered alone or in admixture with particles of other materials. Electrodes
produced using the fine and uniform powders obtained according to the invention have
enhanced stability because of the homogeneity of the particles.
[0047] Another application of the invention is in the production of fine metal powders by
precipitating particles of the oxides (or hydroxides) and subjecting them to reduction
to give a very fine and homogenous powder of the metal(s). This is for instance useful
in producing very fine iron particles which would be pressed into a body for use as
an electrode in batteries such as the iron-air or iron-nickel battery.
[0048] It will be appreciated that the described embodiments of pH control for the precipitation
of particles in an electrolysis cell apply also to electrodeposition cells particularly
where it is desired to obtain cathodic deposits of desired uniform grain size, crystal
structure and porosity all of which may be adversely affected by small fluctuations
in the pH adjacent the depositing zone. Using the described pH control devices, where
the pH is measured in or very close to the depositing zone, uniform cathodic deposits
can be grown to the desired thickness with great simplification of the process control.
Likewise, the electrophoretic deposition of charged colloidal particles is pH dependent
and small fluctuations of the pH affect the rate of deposition and the deposit quality
and uniformity. By controlling the deposition as a function of the pH measured in
or close to the depositing zone according to the invention, the deposit quality can
be improved and the process control simplified.
1. A method of controlling the precipitation or depositing of particles from a solution
in a cell in which an electric field is applied between an anode and a cathode, characterized
by measuring the pH of the solution in the cell using a probe shielded from migrating
electric current and adjusting the pH of the solution to a selected value as a function
of the measured pH.
2. The 'method of claim 1, characterized by supplying a first electrical signal representative
of the measured pH, comparing the first electrical signal with a reference electrical
signal corresponding to the selected pH, and adjusting the pH of the solution as a
function of the difference of the first signal and the reference signal to maintain
it close to the selected pH.
3. The method of claim 1 or 2, wherein the pH of the solution is adjusted by bubbling
acid vapour, air or base vapour into the solution.
4. The method of claim 1, 2 or 3, in which metal compounds are precipitated from a
solution of metal salts obtained by dissolving at least one metal anode.
5. The method of claim 1 or 2, in which metal compounds are precipitated from a preprepared
solution of metal salts introduced into a compartment of the cell in which the particles
are precipitated, and ions liberated by the salts are passed through a separator into
another compartment of the cell by passing an electrolysis current at a rate controlled
as a function of the measured pH to keep the pH of the solution at the selected value.
6. The method of claim 5, wherein ions of at least one further metal to be precipitated
are obtained by dissolving at least one metal anode, and the pH of the solution is
adjusted by bubbling acid vapour, air or base vapour into the solution when said controlled
electrolysis current reaches a threshold value.
7. The method of any preceding claim, wherein there are gas bubbles in the solution
and the pH measurement probe is disposed in a tube which shields the probe from migrating
current and from the gas bubbles.
8. The method of claim 7, wherein an oxidizing gas is supplied to the solution to
precipitate oxide(s) of metal salts dissolved in the solution.
9. The method of claim 1 or 2, wherein an oxidizing agent for precipitation of oxide(s)
of metal(s) of salts dissolved in the solution is generated in situ in the solution
by an insoluble anode.
10. The method of claim 9, wherein the current supplied to said insoluble anode is
controlled as a function of the measured pH.